Obtain the expression for the magnetic energy stored in a solenoid in terms of magnetic field B, area A and length l of the solenoid.

Explanation

From Equation `epsi=-L (Di)/(dT)`
`U_(B)=1/2LI^(2)=1/2L((B)/(mu_(0)n))^(2) (since B=mu_(0)nI "for a solenoid")`
`=1/2(mu_(0)n^(2)Al)((B)/(mu_(0)n))^(2) ["from Equation L="mu_(0)n^(2)Al]`
`=1/(2mu_(0))B^(2)Al`